The present invention relates to a casting method which employs rapid solidification of metal, rare-earth metal, high-melting-point metal, nonmetal or the like, as well as to a casting apparatus and a cast alloy.
In recent years, as peripheral equipment for personal computersxe2x80x94such as HDDs (hard disk drives)xe2x80x94AV equipment, household electric equipment, and the like have become lightweight, compact, and of higher performance, demand for sintered rare-earth magnets represented by Nd-based (neodymium-based) magnets has sharply increased. Typical alloys for such magnets are the Ndxe2x80x94Fexe2x80x94B-type alloys, which additionally contain iron and boron and are typified by a composition Nd2Fe14B.
In many of these rare-earth magnets, in order to improve magnetic properties, to enhance economical efficiency through effective use of rare-earth elements, which are limited resources, and to enhance use-related properties (such as heat resistance and corrosion resistance), Dy (dysprosium), Pr (praseodymium), or similar rare-earth elements are incorporated so as to substitute for some portion of Nd, and Co, Al, Cu, or like elements are incorporated so as to substitute for some portion of Fe.
When there is no particular reason to limit a rare-earth element contained in rare-earth magnets including those mentioned above to Nd, the rare-earth magnets are collectively referred to as R-T-B-type magnets (R: rare-earth element; T: transition metal element).
Generally, all industrially produced R-T-B-type magnets contain R in an amount slightly exceeding the stoichiometric amount for the composition R2T14B. Thus, in a magnet alloy ingot, a phase which contains a rare-earth element(s), represented by R, at high concentration (hereinafter called the R-rich phase) is generated.
The R-rich phase is known to play the following important roles in R-T-B-type magnets.
(1) Since melting point of the R-rich phase is low, the phase becomes a liquid phase during sintering in a magnet production step, thereby contributing to achievement of high density of the resultant magnet and thus to improvement in remanence.
(2) The R-rich phase functions to smoothen grain boundaries, thereby reducing the number of nucleation sites in a reversed magnetic domain. Moreover, being nonmagnetic, the R-rich phase magnetically insulates the main phase, thereby enhancing the coercivity.
(3) Since the R-rich phase expands through absorption of hydrogen, this feature is utilized for decrepitating an ingot into pieces. Specifically, the R-rich phase is caused to absorb hydrogen so as to expand. As a result, cracks are generated within an alloy ingot, thereby decrepitating the ingot into pieces. The R-rich phase serves as a starting point of so-called hydrogen decrepitation.
In recent years, R-T-B-type magnets of improved magnetic characteristics, particularly R-T-B-type magnets of enhanced maximum magnetic energy product (BHmax), have been developed. In order to obtain such a high-performance magnet, the percentage of the R2T14B phase (hereinafter called the T1 phase), which produces magnetism, must be increased, and the R-rich phase must be reduced. In order to fulfill these needs, the total rare-earth element content (hereinafter called the TRE content) must be reduced so as to attain a near stoichiometric composition.
In such a case, the following problems that affect magnetic properties of the produced magnets are involved in alloy production steps and magnet production steps.
First, in melting and casting of an alloy; for example, a ternary alloy of Ndxe2x80x94Fexe2x80x94B, the T1 phase forms through peritectic reaction between a primary xcex3Fe phase and a liquid phase. Thus, as the TRE content (the total R content) decreases, an xcex1Fe phase, which is a transformed form of xcex3Fe, tends to form. The xcex1Fe phase appears in the form of dendrites and extends three-dimensionally within the alloy, thereby significantly deteriorating crushability of the alloy in the magnet production step.
Second, when the TRE content is decreased, the percentage of the existing R-rich phase decreases. Thus, the aforementioned effects exerted by the R-rich phase; i.e., achievement of high density of the resultant magnet and enhancement in coercivity to a magnet, cannot be expected.
In order to solve the above problems, a strip casting process (SC process) has been developed (see, for example, Japanese Patent Application Laid-Open (kokai) Nos. 5-222488 and 5-295490). According to the SC process, a molten alloy is poured onto a water-cooled rotating roller of copper through a tundish and solidifies upon contact with the roll, so as to continuously produce a strip-like ingot. Subsequently, the strip-like ingot is crushed coarsely, and ultimately into flakes.
When an R-T-B-type rare-earth magnet alloy is cast by the SC process, very thin flakes, each having a thickness of about 0.2 mm to 0.4 mm, can be obtained, and therefore, cooling for solidification can be high. Thus, the molten metal can be cooled below a co-existence region of a liquid phase and xcex3Fe. That is, the T1 phase forms directly without formation of xcex3Fe. For example, a ternary alloy of Ndxe2x80x94Fexe2x80x94B can be cast without formation of dendritic xcex1Fe while the Nd content ranges down to about 12.7 at. % (28.5% by mass), at which Nd content a high-performance magnet of 400 kJ/m3 or higher can be produced. (Y. Hirose, H. Hasegawa, S. Sasaki and M. Sagawa, Proceedings of the 15th International Workshop on Rare-Earth Magnets and Their Applications, Volume 1, pages 77-86, 30 Aug.-3 Sep. 1998, Dresden, Germany).
Because of high rate of solidification, an alloy cast by the SC process has a relatively small crystal grain size of 20 xcexcm to 30 xcexcm as measured along the short axis. FIG. 7 schematically shows a cross-sectional structure of an R-T-B rare-earth alloy cast by the SC process and having an R content of 11.8 at. % (26.5% by mass) or more. In FIG. 7, the bottom surface (called the mold contact surface) is the surface of an ingot in contact with a mold, and the top surface (called the free surface) is opposite the mold contact surface.
Excess R over the stoichiometric amount in the composition R2T14B is diffused out from the solidification interface during solidification, thereby generating lamellar R-rich phases 30 arranged at intervals of 3 xcexcm to 10 xcexcm. The R-rich phases 30 form on the grain boundaries 28 of and within a crystal grain 29. As compared with a conventional alloy cast by means of a book-mold, the R-rich phases 30 are distributed finely and uniformly. Thus, crushability during hydrogen decrepitation is significantly improved, such that pulverized particles attain a size which is a fraction of the crystal grain size. That is, it is possible to obtain a powder constituted solely by single-crystal particles. A region denoted by reference numeral 32 is the T1 phase.
A powder consisting of single-crystal particles facilitates, in a later step of compaction in a magnetic field, formation of a compact which is oriented in the direction of the C-axis, which serves as an easy-magnetization axis.
However, mere mechanical pulverization disintegration without involvement of hydrogen decrepitation causes cracking to propagate through grains (i.e., penetrating grains) in the form of cleavage fracture without utilization of the R-rich phases generated on grain boundaries and within grains. As a result, among pulverized particles, an increased number of particles come to have crystal grain boundaries 28, or in other words, are not single crystal particles. Accordingly, the degree of alignment drops at the time of compaction in a magnetic field, causing an impairment in magnetization and a decrease in magnetic energy product after sintering.
The present inventors devised another rapid solidification process and an apparatus therefor (Japanese Patent Application Laid-Open (kokai) Nos. 08-13078 and 08-332557). Specifically, a molten material is introduced into a rotating mold via a box-like tundish, which is disposed in a reciprocative manner inside the mold and has a plurality of nozzles, whereby the molten material is deposited and solidifies on the inner surface of the rotating mold (CC (Centrifugal Casting) process).
In the CC process, a molten material is continuously poured onto an ingot which has already been deposited and solidified. The additionally cast molten material solidifies while the mold makes one rotation; thus, the rate of solidification can be increased. The newly poured molten material and the surface of the existing solidified ingot fuse together, whereby crystals grow epitaxially. Thus, the CC process can produce an alloy whose crystal grain size is several times longer than that of an alloy produced by the SC process.
However, in the production of an alloy of low R content, in contrast to the SC process, the CC process unavoidably involves formation of dendritic xcex1Fe due to low cooling rate in a high-temperature zone. For example, in production of a ternary alloy of Ndxe2x80x94Fexe2x80x94B, formation of dendritic xcex1Fe is observed at an Nd content of about 14.4 at. % (31.5% by mass) or less, which is not observed in the SC process.
When the deposition rate of a molten material is decreased in order to increase the solidification-cooling rate in the CC process, the temperature of the solidified ingot drops, thereby increasing the temperature-dropping rate of the deposit layer of the additional molten material, leading to an increase in the solidification-cooling rate. However, decreasing the deposition rate in the CC process involves the following problems.
(1) Since the deposition rate is a value obtained by dividing the amount of supply (volume of supply) of a molten material per unit time by an effective inner surface area of a mold, the effective area of a mold may be increased. Specifically, a mold of large inside diameter or long length relative to the amount of the material to be cast may be used. However, this causes an increase in equipment scale, requiring a larger chamber. Also, the consumption of inert gas increases. Thus, economical efficiency becomes low.
(2) In order to decrease the deposition rate through decrease in the amount of supply of a molten material, the head of the molten material contained in a tundish must be lowered. In this case, the supply of the molten material becomes nonuniform, causing difficulty in obtaining an ingot of uniform thickness in the longitudinal direction of the mold. Accordingly, the deposition rate of the molten material varies in the longitudinal direction, resulting in nonuniform microstructure of ingot.
(3) When the amount of supply of a molten material is decreased, the temperature of the molten material contained in a tundish drops significantly, causing difficulty in performing stable casting.
(4) When the deposition rate is decreased, a rough surface of ingots tends to form, thus reducing commercial value.
The foregoing discussion is directed to R-T-B-type rare-earth magnet alloys. In recent years, demand for hydrogen storage mischmetal-nickel alloys has increased for use as materials for negative electrodes of nickel-hydride batteries, which are a type of secondary batteries. But regrettably, the hydrogen storage alloys also involve similar problems.
An intermetallic compound which serves as an important component of a hydrogen storage mischmetal-nickel alloy is a compound assuming an M1T5 phase, which contains a mischmetal M, which is a mixture of rare-earth elements, such as Ce, La, Nd, and Pr; and a transition metal T, which includes Ni as a fundamental element, at the ratio 1:5.
The transition metal T includes Ni as a main element as well as additional elements, such as Co, Al, Mn, and Cu, in order to adjust the equilibrium pressure associated with absorption and desorption of hydrogen and to improve catalytic characteristics in application to negative electrodes and various characteristics in application to batteries, such as charge-discharge cycle characteristics.
The M1T5 phase does not involve the problem of dendritic xcex1Fe formation, but, in casting through use of an ordinary book mold, involves the problem that Mn of added elements segregates, causing impairment in charge-discharge cycle characteristics. Thus, as in the case of magnet alloys, a rapid quench method and the SC process are proposed for production of hydrogen storage mischmetal-nickel alloy (Japanese Patent Application Laid-Open (kokai) No. 05-320792).
However, the proposed rapid cooling method involves the problem that residual strain tends to be locked in an ingot, causing impairment in hydrogen storage characteristics.
The present inventors also devised a method for producing hydrogen storage mischmetal-nickel alloys (Japanese Patent Application Laid-Open (kokai) No. 09-180716), making use of the CC process. However, the CC process unavoidably involves segregation of Mn due to slow solidification-cooling rate. A difficulty arises in uniformly melting any of metals of high melting point, such as Ti, Mo, Nb, V, W, Ta, and Cr, and alloys and intermetallic compounds which contain the metal(s).
An object of the present invention is to solve the above-mentioned problems and to provide a casting method and a casting apparatus featuring a solidification-cooling rate higher than that of the conventional CC process. Another object of the invention is to provide, by the casting method, a high-performance R-T-B-type rare-earth magnet alloy, a high-performance hydrogen storage mischmetal-nickel alloy, or the like while formation of the dendritic xcex1Fe phase or formation of a segregation phase of Mn or the like is suppressed. A further object of the invention is to provide a casting method and a casting apparatus for producing an alloy which contains a metal of high melting point, such as Ti, Mo, Nb, V, W, Ta, Cr, or the like, which involves difficulty in casting.
The present inventors conducted extensive studies in an attempt to solve the aforementioned problems, and as a result have attained the present invention.
A centrifugal casting method according to the present invention comprises the steps of pouring a molten material onto a rotary body; sprinkling the molten material by the effect of rotation of the rotary body; and causing the sprinkled molten material to be deposited and to solidify on the inner surface of a rotating cylindrical mold. The axis of rotation R of the rotary body and the axis of rotation L of the cylindrical mold are caused not to run parallel to each other.
Preferably, the rotary body assumes the form of a container having a bottom portion and a sidewall; the sidewall has a hole portion formed therein; and the molten material is poured into an opening portion of the container-like rotary body to thereby sprinkle the molten material through the hole portion.
Preferably, the rotary body and the cylindrical mold rotate in the same direction.
Another centrifugal casting method according to the present invention comprises the steps of melting a metal-containing material body through application of heat while rotating the material body; sprinkling the molten material by the effect of rotation; and causing the sprinkled molten material to be deposited and to solidify on the inner surface of a rotating cylindrical mold. The axis of rotation R of the metal-containing material body and the axis of rotation L of the cylindrical mold are caused not to run parallel to each other.
Preferably, the metal-containing material body and the cylindrical mold rotate in the same direction.
In these centrifugal casting methods, preferably the molten material is subjected to a force of not less than 1 G induced by rotation of the rotary body.
Further preferably, in these centrifugal casting methods, the molten material is subjected to a force of not less than 3 G induced by rotation of the cylindrical mold.
Preferably, the angle of inclination xcex8 formed by the axis of rotation R of the rotary body and the axis of rotation L of the cylindrical mold ranges from 5 degrees to 40 degrees.
Preferably, the angle of inclination xcex8 formed by the axis of rotation R of the metal-containing material body and the axis of rotation L of the cylindrical mold ranges from 5 degrees to 40 degrees.
Preferably, when the molten material is caused to be deposited and to solidify on the inner wall of the cylindrical mold, average deposition rate is not greater than 0.015 cm/sec.
More preferably, when the molten material is caused to be deposited and to solidify on the inner wall of the cylindrical mold, average deposition rate is not greater than 0.010 cm/sec.
Most preferably, when the molten material is caused to be deposited and to solidify on the inner wall of the cylindrical mold, average deposition rate is not greater than 0.005 cm/sec.
Preferably, when the molten material is deposited and solidifies on the inner wall of the cylindrical mold, the average surface temperature of an ingot is 0.4T to 0.8T, wherein T (K) is the solidification starting temperature of the molten material.
A centrifugal casting apparatus according to the present invention comprises a rotatable cylindrical mold; a rotary body disposed within the cylindrical mold; and a supply apparatus for pouring a molten material onto the rotary body. The rotary body is disposed such that the axis of rotation L of the cylindrical mold and the axis of rotation R of the rotary body do not run parallel to each other. The molten material poured onto the rotary body is sprinkled by the effect of rotation of the rotary body and is caused to be deposited and to solidify on the inner wall of the cylindrical mold.
Another centrifugal casting apparatus according to the present invention comprises a rotatable cylindrical mold; a rotation drive mechanism to which a metal-containing material body is attached such that at least an end of the metal-containing material body is located within the cylindrical mold and which is adapted to rotate the metal-containing material body; and a melting apparatus for melting the metal-containing material body through generation of arc or plasma arc. The axis of rotation L of the cylindrical mold and the axis of rotation R of the metal-containing material body are caused not to run parallel to each other. The molten metal-containing material is sprinkled by the effect of rotation of the metal-containing material body and is caused to be deposited and to solidify on the inner wall of the cylindrical mold.
These centrifugal casting apparatuses are preferably such that the angle of inclination xcex8 formed by the axis of rotation L of the cylindrical mold and the axis of rotation R of the rotary body or the angle of inclination xcex8 formed by the axis of rotation L of the cylindrical mold and the axis of rotation R of the metal-containing material body can be varied during deposition of the molten material.
These centrifugal casting apparatuses are preferably such that the cylindrical mold and/or the rotary body, or the cylindrical mold and/or the metal-containing material body, can be reciprocated along the axis of rotation L during deposition of the molten material.
The centrifugal casting method of the present invention is preferably such that the angle of inclination xcex8 formed by the axis of rotation L of the cylindrical mold and the axis of rotation R of the rotary body, or the angle of inclination xcex8 formed by the axis of rotation L of the cylindrical mold and the axis of rotation R of the metal-containing material body, is varied during deposition of the molten material.
Preferably, the cylindrical mold and/or the rotary body is reciprocated along the axis of rotation L during deposition of the molten material.
Preferably, the cylindrical mold and/or the metal-containing material body is reciprocated along the axis of rotation L during deposition of the molten material.
The centrifugal casting method of the present invention is suited for casting a rare-earth magnet alloy.
Preferably, the rare-earth magnet alloy contains as rare-earth elements one or more elements selected from among Nd, Pr, and Dy.
Particularly preferably, the rare-earth magnet alloy contains one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. %.
Further preferably, the rare-earth magnet alloy contains one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.8 at. % to 14.4 at. %.
Most preferably, the rare-earth magnet alloy contains one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.8 at. % to 13.5 at. %.
The present invention is suited for casting an R-T-B-type rare-earth magnet alloy (R: rare-earth elements including at least one or more elements selected from among Nd, Pr, and Dy; and T: transition metals including Fe).
A rare-earth magnet alloy can be produced by heat-treating, at a temperature ranging from 900xc2x0 C. to 1,150xc2x0 C., a rare-earth magnet alloy obtained by the centrifugal casting method of the present invention.
A rare-earth magnet alloy powder can be produced by pulverizing a rare-earth magnet alloy obtained by the centrifugal casting method of the present invention or by heat-treating the rare-earth magnet alloy at a temperature ranging from 900xc2x0 C. to 1,150xc2x0 C., followed by pulverization.
A sintered magnet can be produced from the thus-obtained rare-earth magnet alloy powder.
A magnet powder for use in an anisotropic bonded magnet can be produced by subjecting the thus-obtained rare-earth magnet alloy powder to an HDDR treatment.
An anisotropic bonded magnet can be produced from this magnet powder for use in an anisotropic bonded magnet.
The present invention provides a rare-earth magnet alloy which is obtained through casting, characterized by containing one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. % and characterized in that, when determined in an as-cast state of the alloy, a microstructure containing the dendritic xcex1Fe phase occupies an area percentage of not greater than 10% as measured on a cross section of a cast product taken along a thickness direction and that the cast product assumes a thickness of 3 mm to 30 mm.
The present invention provides a rare-earth magnet alloy which is obtained through casting, characterized by containing one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. % and characterized in that, when determined in an as-cast state of the alloy, a microstructure containing dendritic xcex1Fe occupies an area percentage of not greater than 10% as measured on a cross section of a cast product taken along a thickness direction and that crystal grains having a diameter of not less than 1,000 xcexcm as measured along a long axis occupy an area percentage of 10% to 98% as measured on the cross section.
The present invention provides an R-T-B-type rare-earth magnet alloy which is obtained through casting, characterized in that, when determined in an as-cast state of the alloy, a microstructure containing the dendritic xcex1Fe phase occupies an area percentage of not greater than 10% as measured on a cross section of a cast product taken along a thickness direction and that the cast product assumes a thickness of 3 mm to 30 mm.
The present invention provides an R-T-B-type rare-earth magnet alloy which is obtained through casting, characterized in that, when determined in an as-cast state of the alloy, a microstructure containing dendritic xcex1Fe occupies an area percentage of not greater than 10% as measured on a cross section of a cast product taken along a thickness direction and that crystal grains having a diameter of not less than 1,000 xcexcm as measured along a long axis occupy an area percentage of 10% to 98% as measured on the cross section.
The present invention provides a rare-earth magnet alloy which is obtained through casting, characterized by containing one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. % and characterized in that, when determined in an as-cast state of the alloy, the dendritic xcex1Fe phase is substantially absent as observed on a cross section of a cast product taken along a thickness direction and that the cast product assumes a thickness of 3 mm to 30 mm.
The present invention provides a rare-earth magnet alloy which is obtained through casting, characterized by containing one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. % and characterized in that, when determined in an as-cast state of the alloy, the dendritic xcex1Fe is substantially absent as observed on a cross section of a cast product taken along a thickness direction; crystal grains having a diameter of not less than 1,000 xcexcm as measured along a long axis occupy an area percentage of 50% to 98% as measured on the cross section; and the cast product assumes a thickness of 3 mm to 30 mm.
The present invention provides a rare-earth magnet alloy which is obtained through casting, characterized by containing one or more elements selected from among Nd, Pr, and Dy in a total amount of 11.0 at. % to 15.2 at. % and characterized in that, when determined in an as-cast state of the alloy, the dendritic xcex1Fe is substantially absent as observed on a cross section of a cast product taken along a thickness direction; as measured on the cross section, crystal grains having a diameter of not less than 1,000 xcexcm as measured along a long axis occupy an area percentage of 50% to 98% and crystal grains assume an average diameter of not less than 60 xcexcm as measured along a short axis; and the cast product assumes a thickness of 3 mm to 30 mm.
These rare-earth magnet alloys are preferably such that one or more elements selected from among Nd, Pr, and Dy are contained in a total amount of 11.8 at. % to 14.4 at. %.
These rare-earth magnet alloys are more preferably such that one or more elements selected from among Nd, Pr, and Dy are contained in a total amount of 11.8 at. % to 13.5 at. %.
These rare-earth magnet alloys are preferably such that a cast product assumes a thickness of 5 mm to 20 mm.
The present invention is suited for casting a rare-earth hydrogen storage alloy.
The rare-earth hydrogen storage alloy is preferably a mischmetal-nickel alloy.
Preferably, a metal, an alloy, or an intermetallic compound to be cast has a melting point or a solidification starting temperature of 1400xc2x0 C. or higher.
A metal, an alloy, or an intermetallic compound containing Ti can be cast.
Preferably, the above-mentioned rare-earth magnet alloy serves as a main-phase alloy for use in production of a rare-earth magnet by a two-alloy blending method.
A rare-earth magnet powder is preferably such that the above-mentioned rare-earth magnet alloy powder serves as a main-phase alloy powder for use in production of a rare-earth magnet by a two-alloy blending method.
A sintered magnet is produced from the above-mentioned rare-earth magnet alloy.
Particularly, a sintered magnet is produced through blending of a main-phase alloy powder comprising the above-mentioned rare-earth magnet powder; and a boundary-phase alloy powder containing Nd, Pr, and Dy in a total amount greater than that of Nd, Pr, and Dy contained in the main-phase alloy powder.